Biophysical studies will be continued to ascertain the important factors required for the assembly and disassembly of neurofibrous proteins in the normal, aging and senile brain. Proteins to be studied in vitro include the major constituents of neurotubules, neurofilaments and paired helical filaments (PHF). Neurotubules (microtubules from the brain) and neurofilaments (also called intermediate filaments) will be purified from calf, rat, and human brain. PHF (called twisted tubules until 1976) have been found primarily in abnormal human brain and will be studied using proteins isolated from autopsied brain from patients with microscopically proven senile dementia of the Alzheimer type. Structure and kinetics will be characterized by electron microscopy and optical measurements such as turbidometry. Neurotubule assembly in vitro can be regulated by divalent cations, in particular Mg ions, Ca ions, and Zn ions. A detailed analysis of the effects of these ions as well as other metal ions will be made on the assembly, disassembly and structure of the three neurofibrous proteins. Atomic absorption spectrometry, binding studies, nuclear magnetic resonance spectrometry and organotypic cultures of nervous tissue will also be used to study the role of ions. The concentration and assembly of tubulin will be compared in mature rats (12-14 mo.) and old rats (24-26 mo.) of the Fisher strain.